This invention relates to an optical amplifier circuit for directly amplifying input signal light.
In order to directly amplify input signal light, an optical amplifier circuit comprises an excitation light source for generating excitation light, a wave combiner for combining the excitation light with the input signal light to produce combined light, an optical fiber doped with a rare-earth element, such as Neodymium, Erbium, or Ytterbium, to be excited by the excitation light. Generally, the excitation light is regarded as polarized light. The optical fiber amplifies the input signal light included in the combined light into amplified output light.
In the optical fiber, the rare-earth element is excited by the excitation light. On the other hand, the input signal light is amplified by stimulated emission effect of the rare-earth element excited by the excitation light while the input signal light passes through the optical fiber. Such as optical amplifier circuit is disclosed in Japanese Unexamined Patent Publication No. 289829/1992 (Tokkai Hei 4-289829).
The wave combiner has a loss dependent upon a polarization state of the excitation light. In other words, the loss in the wave combiner varies with the polarization state of the excitation light. The polarization state varies with environment condition, such as wavelength of the excitation light, ambient air temperature, or vibration effected on the optical amplifier circuit. This means that the strength of the excitation light delivered from the wave combiner varies with the polarization state. Such a variation of the strength of the excitation light causes a variation of an amplification degree of the input signal light because an excitation state in the optical fiber varies with the strength of the excitation light. For the reason, a conventional optical amplifier circuit is unstable in amplification degree.